For example, in a method of modulating a transmission signal by using a plurality of carrier waves, such as OFDM (Orthogonal Frequency Division Multiplex) and W-CDMA (Wideband Code Division Multiple Access), there may be a case where phases of carrier waves overlap each other, thereby causing the transmission signal to have a large peak power.
On the other hand, although a superior linearity is required for a power amplifier, when a signal having a power exceeding a maximum output level is inputted, the output is saturated and nonlinear distortion increases.
Thus, when a signal having a large peak power is inputted in a nonlinear amplifier, nonlinear distortion occurs in an output signal, which causes degradation of reception characteristics on a receiver side and out-of-band radiation.
In order to prevent nonlinear distortion from increasing relative to a peak power, a power amplifier having a wide dynamic range is necessary. However, if the dynamic range of an amplifier is widened for a peak power that does not occur so often, the ratio of a peak power in a short time to an average power of a waveform on a time axis (PAPR: Peak to Average Power Ratio) increases, and thus, the power efficiency decreases.
Therefore, with respect to a signal having a large peak power that does not occur so often, it is more reasonable to suppress the peak power before the signal is inputted to the amplifier, than to directly input the signal to the amplifier. Thus, there exists a device that performs a clipping process that instantaneously applies, in order to suppress a peak power of an IQ baseband signal before its power is amplified, an amplitude in an inverse direction to the IQ baseband signal having a peak power exceeding a predetermined threshold.
Since such a clipping process is a process that applies an impulse-shaped signal in an inverse direction on the time axis, the clipping process is equivalent to applying a noise of a wide frequency band on the frequency axis. This causes a problem that a noise occurs outside the frequency band in a case where only a clipping process is simply performed.
Therefore, for coping with the problem of the out-of-band radiation, there are known peak power suppressing circuits referred to as NS-CFR (Noise Shaping-Crest Factor Reduction) and PC-CFR (Peak Cancellation-Crest Factor Reduction).
Of these, the NS-CFR circuit limits, with respect to a peak component (an increment from a threshold) of an IQ baseband signal having an instantaneous power exceeding the threshold, a frequency band by performing filtering with a low pass filter, a FIR (Finite Impulse Response) filter, or the like, and subtracts the peak component after the frequency band has been limited, from the original IQ baseband signal (see Patent Literature 1).
In the PC-CFR circuit, a cancellation pulse (basic function waveform) that prevents out-of-band radiation even when a clipping process is performed is set in advance, and a cancellation signal obtained by multiplying, by the cancellation pulse, a peak component (an increment from a threshold) of an IQ baseband signal having an instantaneous power exceeding the threshold, is subtracted from the original IQ baseband signal (see Patent Literatures 2 and 3).